How do I customize default error message from spring @Valid validation?

If you want full control over the response message in every controller write a ControllerAdvice. For example, that example transform MethodArgumentNotValidException into a custom json object:

import org.springframework.core.Ordered;
import org.springframework.core.annotation.Order;
import org.springframework.validation.BindingResult;
import org.springframework.web.bind.MethodArgumentNotValidException;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.bind.annotation.ResponseStatus;

import java.util.ArrayList;
import java.util.List;

import static org.springframework.http.HttpStatus.BAD_REQUEST;

/**
 * Kudos http://www.petrikainulainen.net/programming/spring-framework/spring-from-the-trenches-adding-validation-to-a-rest-api/
 *
 */
@Order(Ordered.HIGHEST_PRECEDENCE)
@ControllerAdvice
public class MethodArgumentNotValidExceptionHandler {

    @ResponseStatus(BAD_REQUEST)
    @ResponseBody
    @ExceptionHandler(MethodArgumentNotValidException.class)
    public Error methodArgumentNotValidException(MethodArgumentNotValidException ex) {
        BindingResult result = ex.getBindingResult();
        List<org.springframework.validation.FieldError> fieldErrors = result.getFieldErrors();
        return processFieldErrors(fieldErrors);
    }

    private Error processFieldErrors(List<org.springframework.validation.FieldError> fieldErrors) {
        Error error = new Error(BAD_REQUEST.value(), "validation error");
        for (org.springframework.validation.FieldError fieldError: fieldErrors) {
            error.addFieldError(fieldError.getField(), fieldError.getDefaultMessage());
        }
        return error;
    }

    static class Error {
        private final int status;
        private final String message;
        private List<FieldError> fieldErrors = new ArrayList<>();

        Error(int status, String message) {
            this.status = status;
            this.message = message;
        }

        public int getStatus() {
            return status;
        }

        public String getMessage() {
            return message;
        }

        public void addFieldError(String path, String message) {
            FieldError error = new FieldError(path, message);
            fieldErrors.add(error);
        }

        public List<FieldError> getFieldErrors() {
            return fieldErrors;
        }
    }
}

I know this is kind of old question,

But I just run into it and I found some pretty good article which has also a perfect example in github.

Basically it uses @ControllerAdvice as Spring documentation suggests.

So for example catching 400 error will be achieved by overriding one function:

@ControllerAdvice
public class CustomRestExceptionHandler extends ResponseEntityExceptionHandler {

    @Override
    protected ResponseEntity<Object> handleMethodArgumentNotValid(final MethodArgumentNotValidException ex, final HttpHeaders headers, final HttpStatus status, final WebRequest request) {
        logger.info(ex.getClass().getName());
        //
        final List<String> errors = new ArrayList<String>();
        for (final FieldError error : ex.getBindingResult().getFieldErrors()) {
            errors.add(error.getField() + ": " + error.getDefaultMessage());
        }
        for (final ObjectError error : ex.getBindingResult().getGlobalErrors()) {
            errors.add(error.getObjectName() + ": " + error.getDefaultMessage());
        }
        final ApiError apiError = new ApiError(HttpStatus.BAD_REQUEST, ex.getLocalizedMessage(), errors);
        return handleExceptionInternal(ex, apiError, headers, apiError.getStatus(), request);
    }
}

(ApiError class is a simple object to hold status, message, errors)

One way to do it is adding message in @NotNull annotation on entity properties. And adding @Valid annotation in controller request body.

DTO:

public class User {
   
    @NotNull(message = "User name cannot be empty")
    private String name;

    @NotNull(message = "Password cannot be empty")
    private String password;

    //..
}

Controller:

@RequestMapping(value = "/user", method = RequestMethod.POST)
public ResponseEntity<String> saveUser(@Valid @RequestBody User user) {
    //..
    return new ResponseEntity<>(HttpStatus.OK);
}
// Add one 
@ExceptionHandler(MethodArgumentNotValidException.class)
public ResponseEntity<List<YourErrorResponse>> handleException(MethodArgumentNotValidException ex) {
// Loop through FieldErrors in ex.getBindingResult();
// return *YourErrorReponse* filled using *fieldErrors*
}

You can perform validation with Errors/BindingResult object. Add Errors argument to your controller method and customize the error message when errors found.

Below is the sample example, errors.hasErrors() returns true when validation is failed.

@RequestMapping(value = "/user", method = RequestMethod.POST)
@ResponseBody
public ResponseEntity<String> saveUser(@Valid @RequestBody User user, Errors errors) {
    if (errors.hasErrors()) {
        return new ResponseEntity(new ApiErrors(errors), HttpStatus.BAD_REQUEST);
    }
    return new ResponseEntity<>(HttpStatus.OK);
}